How to edit a slur

In the Score Editor, select the arrow tool in the toolbar (the first
one on the left). Make sure the symbol object is selected in the object
palette. Select a slur by double-clicking on the start of the slur. The
Edit menu will appear. Choose the symbol editing operation you would like.

If you select Edit from the Edit menu, three little diamonds will appear
at the left, right and center of the slur. You use these control points
to shape and move the slur. Click on a control point to select it and
drag it around to shape or move the slur. Alternatively you can move a
control point around using the arrow keys. You can move between control
points by using the TAB and the SHIFT+TAB keys as well as by clicking
on them with the mouse. The middle control point is used for adjusting
the shape of the slur and the outer control points for setting the start
and end points of the slur. When you are finished shaping and positioning
your slur, click the right mouse button or press the ENTER key to enter
it into the score. If you don't want to accept the edits you've made,
press the ESCAPE key.

When you are editing the slur symbol, you can automatically position
the left slur point over the closest note by pressing Z. You can automatically
position the right slur point over the closest note by pressing X. You
can automatically center the slur by pressing V. If the slur is above
the notes, the slur end points go over the closest notes when you press
Z and X. If the slur is below the notes, the slur end points go under
the closest notes when you press Z and X.

You can also erase or move symbols such as a slur using the other tools
in the toolbar. Make sure the symbol object is selected in the object
palette. Select the tool you want and apply the edit by clicking on the
beginning of the slur (to erase using the eraser tool) or by dragging
the slur up, down, left or right.